To observe diffraction, the size of an obstacle

The correct answer is Option (1) → Should be of the same order as wavelength

Diffraction is the bending of light around the edges of an obstacle or aperture. The angular spread is of the order of \( \lambda/a \), where \( a \) is the size of the obstacle, so the effect is noticeable only when \( a \) is comparable to the wavelength.

Option (1) Should be of the same order as wavelength — Correct. Then \( \lambda/a \) is large enough for the bending to be clearly seen.

Option (2) Should be much larger than the wavelength — Incorrect. For an obstacle much larger than the wavelength, \( \lambda/a \) is tiny and the shadow is practically sharp; this is why ray optics works for everyday objects.

Option (3) has no relation with wavelength — Incorrect. The spread depends on \( \lambda/a \), so the size relative to the wavelength is exactly what matters.

Option (4) should be exactly half of the wavelength — Incorrect. No exact size is required; any size of the same order as the wavelength gives clear diffraction.

Note: A similar question was also asked in CUET (UG) 2023 on 21-05-2023 (Slot 3).

NCERT (2026-27):
Diffraction – “If we look clearly at the shadow cast by an opaque object, close to the region of geometrical shadow, there are alternate dark and bright regions just like in interference. This happens due to the phenomenon of diffraction. Diffraction is a general characteristic exhibited by all types of waves, be it sound waves, light waves, water waves or matter waves. Since the wavelength of light is much smaller than the dimensions of most obstacles; we do not encounter diffraction effects of light in everyday observations.”